[volt-nuts] HP-419 and Fluke 845 Modifications

Randy Evans randyevans2688 at gmail.com
Mon Dec 1 14:40:36 EST 2014


The HP-419A is driving the need for an isolated recording output (it
doesn't have one in the basic unit) and I wanted to provide one.    The
circuit is being designed to work with either the HP-419A or the Fluke
845A.  The isolation circuit does not have to be used if not wanted since
it is a completely separate circuit and not needed for the meter to work.
I still need to work on the final HP-845A modifications and i will look at
your suggestions.


On Sun, Nov 30, 2014 at 6:37 PM, Dave M <dgminala at mediacombb.net> wrote:

> Randy,
> Thanks for continuing with this modification.
> Speaking of the Fluke 845A;
> At first glance, you could probably leave the existing power supply
> inverter circuit intact, as well as the isolation transformer for the
> recorder output, with the recorder output transformer being driven (at the
> center tap) by the output of the last DC amp stage of the null amp
> circuit.  That would maintain the isolation of the meter and recorder
> output, and eliminate the need for an extra amp for them.  It would also
> maintain the guard integrity, keeping the instrument's operational
> characteristics as close to original as possible.
> By leaving the existing inverter circuit in place, you can easily derive
> any voltages (+ and -) that your mod needs from it, without the need for
> additional isolated power supplies
> Dave M
> Randy Evans wrote:
>> I finished the H-419A/Fluke 845A design modifications but I am still
>> trying to decide on the preferred power supply design.  The basic
>> meter circuit uses the LTC2054 and LTC6255 with two CR123 Li-Ion 3 V
>> batteries.  The
>> basic meter circuit draws less than 0.2 mA and with 1500 mAH CR123A
>> batteries should be able to go several thousand hours on a set of
>> batteries.   For the meter circuit, it is always battery powered and
>> doesn't have to be plugged into the mains.
>> I also added a TI  AMC1100 isolation amplifier for recording output
>> but it needs a 3.3 VDC isolated supply and a 5VDC output supply.  The
>> current design  uses separate isolation power supplies using  LT8300
>> ICs that work off the mains transformer so there would be no worry
>> about failing batteries.  Therefore, to use the recorder, the unit
>> would have to be plugged in. I am trying to decide if it would be
>> simpler to use batteries for the isolation circuit as well. If I use
>> batteries, do I use rechargeables or non-rechargeable batteries such
>> as the CR123s?  The
>> circuit would need one set for the input isolated circuit side and
>> another set for the recorder output side and the batteries would not
>> last as long
>> as the meter circuit since the isolation circuits draw about 12
>> milliamps. They would still work about one hundred hours most likely
>> and would not
>> draw any current unless the isolation circuit is turned on (with a
>> separate toggle switch).  My preference is to stay with the LT8300
>> power supplies
>> but I thought I would query the group.
>> Any thoughts on what would be your preferences?
>> Thanks,
>> Randy Evans
>> On Mon, Aug 4, 2014 at 8:33 AM, Randy Evans <randyevans2688 at gmail.com>
>> wrote:
>>  I am looking at modifying my (defective) HP419A and Fluke 845A Null
>>> meters using the LTC2054 zero drift op amp.  I am planning on using
>>> a single 4.2 V Lithium Ion battery to power them (no AC input
>>> required and may not include, TBD).  The LTC2054 has a very low bias
>>> current of typically 1pA at room temp and the bias plots vs. temp
>>> show no appreciable increase until about 50C.  Conceptually, it
>>> looks pretty straight forward and, based on my Fluke 887A
>>> modifications, likely will be very stable.
>>> Since this is a very astute group, does anyone have any comments on
>>> the feasibility of this modification before I get too involved?
>>> Regards,
>>> Randy Evans
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